メチルコバラミン
Methylcobalamin/ja
メチルコバラミン（Methylcobalamin, メコバラミン、MeCbl、MeB12）は、[[Vitamin B12|ビタミンB12]の一種であるコバラミンである。］ コバルトのシアノ基がメチル基に置換されている点でシアノコバラミンとは異なる。メチルコバラミンは八面体のコバルト(III)中心を持ち、明るい赤色の結晶として得られる。 配位化学の観点からは、メチルコバラミンが金属-アルキル結合を含む化合物の珍しい例として注目される。ニッケル-メチル中間体は、メタン生成の最終段階として提案されている。
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| Trade names | Cobolmin |
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| Routes of administration | 口から, 舌下, 注射 |
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| Formula | C63H91CoN13O14P |
| Molar mass | 1344.405 g·mol−1 |
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メチルコバラミンは生理学的にはビタミンB12と同等であり、ビタミンB12の摂取不足から生じる病態（ビタミンB12欠乏症）の予防や治療に用いることができる。
Methylcobalamin is also used in the treatment of peripheral neuropathy, diabetic neuropathy, and as a preliminary treatment for amyotrophic lateral sclerosis.
Methylcobalamin that is ingested is not used directly as a cofactor, but is first converted by MMACHC into cob(II)alamin. Cob(II)alamin is then later converted into the other two forms, adenosylcobalamin and methylcobalamin for use as cofactors. That is, methylcobalamin is first dealkylated and then regenerated.
According to one author, it is important to treat vitamin B12 deficiency with hydroxocobalamin or cyanocobalamin or a combination of adenosylcobalamin and methylcobalamin, and not methylcobalamin alone.
Production
Methylcobalamin can be produced in the laboratory by reducing cyanocobalamin with sodium borohydride in alkaline solution, followed by the addition of methyl iodide.
Functions
This vitamer, along with adenosylcobalamin, is one of two active coenzymes used by vitamin B12-dependent enzymes and is the specific vitamin B12 form used by 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR), also known as methionine synthase.
Methylcobalamin participates in the Wood-Ljungdahl pathway, which is a pathway by which some organisms utilize carbon dioxide as their source of organic compounds. In this pathway, methylcobalamin provides the methyl group that couples to carbon monoxide (derived from CO2) to afford acetyl-CoA. Acetyl-CoA is a derivative of acetic acid that is converted to more complex molecules as required by the organism.
Methylcobalamin is produced by some bacteria. It plays an important role in the environment, where it is responsible for the biomethylation of certain heavy metals. For example, the highly toxic methylmercury is produced by the action of methylcobalamin. In this role, methylcobalamin serves as a source of "CH3+".
A lack of cobalamin can lead to megaloblastic anemia and subacute combined degeneration of the spinal cord.